Molecular beam deposition may be broadly defined as a method of growing films, under ultrahigh vacuum conditions, by directing one or more molecular beams at a substrate. A narrower term, which perhaps more accurately describes most current work, is molecular beam epitaxy (MBE) which refers to epitaxial film growth on single crystal substrates by a process that typically involves either the reaction of one or more molecular beams with the substrate or the deposition on the substrate of the beam particles. The term "molecular beam" refers to beams of monoatomic species as well as polyatomic species. The term molecular beam deposition thus includes epitaxial growth as well as includes nonepitaxial growth processes. For example, molecular beam deposition includes the growth of layers of polycrystalline GaAs or amorphous silicon on substrates. Molecular beam deposition is of great interest in present technology because of both advantages it offers in fabricating some types of devices and desires to better understand mechanisms of crystal growth.
Molecular beam deposition, which may be thought of as a variation of simple vacuum evaporation, offers better control over the species incident on the substrate than does vacuum evaporation. Good control over the incident species, coupled with the slow growth rates that are possible, permits the growth of thin layers having compositions, including dopant concentrations, that are precisely defined. Compositional control is aided by the fact that growth is generally at relatively low substrate temperatures, as compared to other growth techniques such as liquid phase epitaxy or chemical vapor deposition, and diffusion processes are very slow. Essentially arbitrary layer compositions and doping profiles may be obtained with precisely controlled layer thicknesses. In fact, layers as thin as a monolayer are grown by MBE. Furthermore, the relatively low growth temperature may permit growth of materials and use of substrate materials that could not be used with higher temperature growth techniques because interdiffusion would degrade desired compositional properties.
Molecular beam epitaxy has been used to fabricate films or layers of numerous semiconductor materials including Group IV and III-V materials as well as Group II-VI and IV-VI materials. Within the III-V materials system, devices such as IMPATT diodes, microwave mixer diodes, double heterostructure junction lasers and superlattice devices have been fabricated. Molecular beam epitaxy of elemental materials, e.g., Si, has developed more recently and devices such as p-n, p-i-n and varactor diodes have been made as well as MOSFET structures. Interest in Si MBE has increased recently because of the development of silicon-metal silicide heterostructures and improved silicon on insulator overgrowth.
Successful exploitation of MBE has, of course, required the development of new apparatus. This development has reached the state where commercial MBE apparatus is now available. However, such apparatus and the research and development apparatus also in use generally have limited processing capabilities. For example, depositing from a large number of molecular beams and processing large numbers of wafers in a relatively short time period are usually not possible. The compositions that may be grown and the output of the apparatus are thus limited.
This limited processing capability arises from many factors. The most important factors include apparatus that permits deposition on only a single substrate at a given time and uses relatively cumbersome methods to transport a substrate between chambers, e.g., between the growth chamber and an analytical chamber that is used to house instruments that are used to characterize the substrate surface, or move a substrate from a growth to an analytical station. Further, the need for a uniform composition over the substrate typically requires that the flux from each oven be constant over the substrate and this requirement thus limits the number of ovens that can be used simultaneously. Additionally, lack of component standardization between, e.g., analytical and growth chambers, limits apparatus flexibility and increases cost.
Recent developments have improved processing capability. For example, U.S. Pat. No. 4,137,865 issued to Alfred Y. Cho on Feb. 6, 1979 describes a molecular beam apparatus for the sequential deposition of material on a plurality of substrates, i.e., deposition on one substrate commences and terminates before deposition on the following substrate commences. An experimental molecular beam apparatus explicitly designed for silicon is schematically illustrated in Journal of Crystal Growth, 45, pp. 287-291, Proceedings of the Fourth International Conference on Vapor Growth and Epitaxy, Nagoya, Japan, July 9-13, 1978. The apparatus has only a single chamber and can process only a single substrate at a time, that is, only one substrate is ever positioned in the chamber at one time. Another apparatus explicitly designed for silicon is described in Journal of Applied Physics, 48, pp. 3345-3399, August, 1977. While this apparatus is well suited for silicon MBE, it also can process only a single substrate at a time.
Further, the use of a vertical deposition geometry may result in flakes of accumulated material falling back into the molecular beam sources. If this occurs, the flake can either contaminate the source or evaporate so rapidly that crystalline defects are created in the growing epitaxial layer. To avoid these undesirable results, Group III-V MBE apparatus using Knudsen effusion sources are generally tilted from the vertical to permit near-horizontal deposition. Such tilting is not possible with the electron beam evaporation sources generally used for Si MBE.